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Major-element chemical analyses of 130 samples of high-grade plagioclase gneiss, together with geologic data, suggest that metadacitic gneisses of the late Proterozoic Avalon and Putnam-Nashoba terranes include four extrusive and two intrusive units formerly mapped as Monson Gneiss and Middletown Formation. Evidence for an igneous rather than sedimentary protolith for these gneisses comes from the mineralogic and chemical homogeneity, and the consistency of the chemical variations within individual bodies of gneiss with fractional crystallization processes. Evidence for the extrusive origin of some units comes from interlayered contacts with metasediments and metarhyolitic (alaskitic) and metatholeiitic (amphibolitic) volcanics, and from a 20- to 50-m-scale chemical cyclicity, typical of modern ash-flow tuffs. Evidence for an intrusive origin of two bodies comes from abundant amphibolite and calc-silicate xenoliths, from a massive structure lacking compositional layering, and from kilometer-scale chemical zoning or homogeneity.

The identification of some units as lithodemic orthogneisses indicates that intrusion as well as extrusion led to the lithologic sequences in the Avalon terrane of southeastern Connecticut. It also precludes the lithostratigraphic correlation of these orthogneisses with paragneisses. Given this geochemical and geologic evidence, we believe that previously advanced stratigraphic arguments that major recumbent folds relate gneiss bodies now interpreted to be intrusive and extrusive should be reexamined. We believe that the part of the Avalon terrane consisting of metavolcanic rocks may be a right-side-up volcanic pile, and that major recumbent folding may not be present.

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